feat(medios,secciones): application layer + handlers TDD — ADM-001 B3+B4

- IMedioRepository, ISeccionRepository interfaces
- MediosQuery, SeccionesQuery common records
- TipoSeccion static AllowedTipos helper
- Medios: 6 use cases (Create/Update/Deactivate/Reactivate/List/GetById) with validators, handlers and DTOs
- Secciones: 6 use cases mirroring Medios; Create validates MedioId active via IMedioRepository
- 52 unit tests (xUnit + NSubstitute) all green; audit LogAsync asserted per mutating handler
- DI registrations for all 12 handlers and validators auto-scanned via AddValidatorsFromAssemblyContaining

tests/SIGCM2.Application.Tests/Medios/GetById/GetMedioByIdQueryHandlerTests.cs

@@ -0,0 +1,46 @@
+using NSubstitute;
+using SIGCM2.Application.Abstractions.Persistence;
+using SIGCM2.Application.Medios.GetById;
+using SIGCM2.Domain.Entities;
+using SIGCM2.Domain.Exceptions;
+
+namespace SIGCM2.Application.Tests.Medios.GetById;
+
+public class GetMedioByIdQueryHandlerTests
+{
+    private readonly IMedioRepository _repo = Substitute.For<IMedioRepository>();
+    private readonly GetMedioByIdQueryHandler _handler;
+
+    public GetMedioByIdQueryHandlerTests()
+    {
+        _handler = new GetMedioByIdQueryHandler(_repo);
+    }
+
+    private static Medio MakeMedio(int id) =>
+        new(id, "COD" + id, "Nombre " + id, TipoMedio.Radio, 10, true, DateTime.UtcNow, null);
+
+    [Fact]
+    public async Task Handle_NotFound_ThrowsMedioNotFoundException()
+    {
+        _repo.GetByIdAsync(999, Arg.Any<CancellationToken>()).Returns((Medio?)null);
+
+        await Assert.ThrowsAsync<MedioNotFoundException>(
+            () => _handler.Handle(new GetMedioByIdQuery(999)));
+    }
+
+    [Fact]
+    public async Task Handle_HappyPath_ReturnsDtoWithCorrectFields()
+    {
+        var medio = MakeMedio(3);
+        _repo.GetByIdAsync(3, Arg.Any<CancellationToken>()).Returns(medio);
+
+        var result = await _handler.Handle(new GetMedioByIdQuery(3));
+
+        Assert.Equal(3, result.Id);
+        Assert.Equal("COD3", result.Codigo);
+        Assert.Equal("Nombre 3", result.Nombre);
+        Assert.Equal(TipoMedio.Radio, result.Tipo);
+        Assert.Equal(10, result.PlataformaEmpresaId);
+        Assert.True(result.Activo);
+    }
+}